1. Field of the Invention
The present invention relates to a power supply control circuit for controlling a current which is supplied from a power source to a load by controlling a transistor disposed between the power source and the load.
2. Description of the Related Art
Recently, many mobile electronic instruments have been used, and the mobile electronic instrument is driven by a secondary battery such as a lithium-ion battery. The secondary battery installed in the mobile electronic instrument is charged from a DC (direct current) power source by connecting to the DC power source via an AC adaptor or a USB port.
In order to control a charging current, a charging control circuit is disclosed. The charging control circuit charges a secondary battery by controlling a current which is supplied from a DC power source to the secondary battery by being connected between the DC power source and the secondary battery such as the lithium-ion battery (refer to Patent Documents 1 and 2).
FIG. 2 is a circuit diagram including a charging control circuit.
In FIG. 2, a DC voltage is applied to a charging control circuit 11 from a DC power source 12. The DC voltage is smoothened by a capacitor C1 and is applied to the source of a control transistor M1. The control transistor M1 is a p channel MOS transistor and its drain is connected to a secondary battery 13 via a current detecting resistor Rs. A starting rheostat R1 is connected between the source and the gate of the control transistor M1. A capacitor C2 is connected to the secondary battery 13 in parallel and smoothens a voltage applied to the secondary battery 13.
The control transistor M1 controls a current which is supplied to the secondary battery 13 from the DC power source 12 by being controlled corresponding to a control signal from a control circuit 21. A voltage at both the ends of the current detecting resistor Rs is applied to the control circuit 21. The control circuit 21 controls the control transistor M1 so that a voltage applied to the secondary battery 13 becomes a constant voltage or a current applied to the secondary battery 13 becomes a constant current.
Between the drain and the back gate of the control transistor M1, a body diode D1 is formed as a plastic diode so that a current from the secondary battery 13 to the DC power source 12 becomes a forward current. Therefore, when the voltage of the secondary battery 13 becomes larger than that of the DC power source 12, a current flows reversely from the secondary battery 13 to the DC power source 12 via the body diode D1.
In order to prevent the reverse current, a diode (not shown) is disposed in series between the DC power source 12 and the secondary battery 13 so that a current from the DC power source 12 to the secondary battery 13 becomes a forward current (refer to Patent Document 1). Further, in order to prevent the reverse current, a voltage applied to the back gate of the control transistor M1 is controlled (refer to Patent Document 2).
[Patent Document 1] Japanese Laid-Open Patent Application No. 11-69649
[Patent Document 2] Japanese Laid-Open Patent Application No. 2001-51735
However, when the diode is disposed in series between the DC power source 12 and the secondary battery 13, a voltage drop caused by the forward current in the diode occurs. Consequently, the charging efficiency is decreased.